Active magnetic regenerative processes and systems employing hydrogen as heat transfer fluid and process

ABSTRACT

A system including:
         an active magnetic regenerative refrigerator apparatus that includes a high magnetic field section in which a hydrogen heat transfer fluid can flow from a cold side to a hot side through at least one magnetized bed of at least one magnetic refrigerant, and a low magnetic field or demagnetized section in which the hydrogen heat transfer fluid can flow from a hot side to a cold side through the demagnetized bed;   a first conduit fluidly coupled between the cold side of the low magnetic field or demagnetized section and the cold side of the high magnetic field section; and   a second conduit fluid coupled to the first conduit, an expander and at least one liquefied hydrogen storage module.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 15/937,773filed Mar. 27, 2018, which claims the benefit of U.S. ProvisionalApplication No. 62/477,928, filed Mar. 28, 2017, which is hereinincorporated by reference in its entirety.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No.DE-AC05-76RL01830 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND

It is well known that several types of liquefiers use compression andexpansion of the same gas as the internal heat transfer gas and theprocess gas. For example, in a simple Linde cycle liquefier, a gas to beliquefied is compressed to high pressure near room temperature followedby rejection of the heat of compression into an ambient heat sink. Thecompressed gas is cooled in a counter-flow heat exchanger to a coldertemperature near but higher than the liquefaction temperature of the gasbefore it is isenthalpically expanded to a low pressure such that aportion of the gas liquefies. The vapor fraction of the cold gas isreturned through the counterflow heat exchanger to the suction side ofthe compressor. The liquid yield in the Linde cycle is relatively smalland its FOM, a measure of liquefier efficiency, may only be about 0.10.The mass flow of the gas that is liquefied is made-up at the suctionside of the compressor by a feed gas supply input. Other more efficientliquefier designs such as the Claude-cycle or Collins-cycle aresignificantly more complex than the Linde cycle and can achieve FOMs ashigh as 0.35 by using the working gas both as the heat transfer gas andthe process gas. There are several gas cycle refrigerators that are usedas liquefiers where the heat transfer gas or working gas producing thecooling is separate from the process gas that is different from the heattransfer gas. A good example is a pulse tube refrigerator with heliumgas as its working refrigerant gas and internal heat transfer gas whichis also used to cool and liquefy process gases such as nitrogen orhydrogen through external, separate process gas heat exchangers. To dateactive magnetic regenerative refrigerators (AMRRs) for cryogenicliquefaction use separate heat transfer gas and process gas. Forexample, in previous magnetocaloric hydrogen liquefier designs, heliumgas is used as the heat transfer gas and hydrogen gas is the processstream. Multiple other gases have been suggested as suitable heattransfer gas for higher temperature cryogens. The distinction betweenworking heat transfer gas and process gas is much more obvious in amagnetic cycle because the working refrigerants are solid magneticmaterials instead of a gas.

SUMMARY

Disclosed herein is a process for liquefying a hydrogen gas comprising:

introducing a hydrogen heat transfer fluid into an active magneticregenerative refrigerator apparatus that comprises (i) a high magneticfield section in which the hydrogen heat transfer fluid flows from acold side to a hot side through at least one magnetized bed of at leastone magnetic refrigerant, (ii) a first no heat transfer fluid flowsection in which the bed is demagnetized, (iii) a low magnetic field ordemagnetized section in which the hydrogen heat transfer fluid flowsfrom a hot side to a cold side through the demagnetized bed, and (iv) asecond no heat transfer fluid flow section in which the bed ismagnetized;

continuously introducing the hydrogen heat transfer fluid from the coldside of the low magnetic field or demagnetized section into the coldside of the high magnetic field section;

continuously separating a portion of the cold hydrogen heat transferfluid flowing from the cold side of the low magnetic field ordemagnetized section into an expander; and

isenthalpically expanding the separated portion of the hydrogen heattransfer fluid to produce liquefied hydrogen.

Also disclosed herein is a system comprising:

an active magnetic regenerative refrigerator apparatus that comprises(i) a high magnetic field section in which a hydrogen heat transferfluid can flow from a cold side to a hot side through at least onemagnetized bed of at least one magnetic refrigerant, (ii) a first noheat transfer fluid flow section in which the bed can be demagnetized tocool the magnetic refrigerant by the magnetocaloric effect, (iii) a lowmagnetic field or demagnetized section in which the hydrogen heattransfer fluid can flow from a hot side to a cold side through thedemagnetized bed, and (iv) a second no heat transfer fluid flow sectionin which the bed can be magnetized to heat the magnetic refrigerants bythe magnetocaloric effect;

a first conduit fluidly coupled between the cold side of the lowmagnetic field or demagnetized section and the cold side of the highmagnetic field section; and

a second conduit fluid coupled to the first conduit, an expander and atleast one liquefied hydrogen storage module (e.g., a vessel).

Further disclosed herein is a method for making a magnetic refrigerantcomposition, comprising:

contacting magnetic refrigerant material particles having a largestcross section dimension of up to 250 μm with a binder;

curing the binder; and

bonding an ortho H₂ to para H₂ catalyst to the bonded magneticrefrigerant material particles.

The foregoing will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a cross-section of a rotary wheelembodiment of a single-stage active magnetic regenerative refrigerator(AMRR) with a hydrogen heat transfer fluid and hydrogen process fluid.For example, the embodiment shown in FIG. 1 is a schematic diagram of asingle stage AMRR with layered magnetic materials and hydrogen heattransfer fluid to continuously cool and liquefy a hydrogen processstream.

FIG. 2 is a graph showing temperature and field dependent heat capacityof ferromagnetic refrigerants.

FIG. 3 is a schematic diagram of an active magnetic regenerativeliquefier (AMRL) that includes more than one single-stage activemagnetic regenerative refrigerator (AMRR) coupled together in series.

DETAILED DESCRIPTION

An active magnetic regenerative liquefier (AMRL) may be able to increasethe FOM of cryogenic liquefiers by a factor of 2 over comparableconventional gas cycle liquefiers for decreased energy use and capitalcost. In current AMRL designs the heat transfer fluid and the processfluid are separated in process heat exchangers and are usually differentstreams. Such separation results in two sub-systems to cool and liquefythe process gas. Elimination of the process heat exchangers, associatedpiping, instrumentation, and pumping power would decrease the capitalcost and the pumping power.

Disclosed herein are processes and systems that include active magneticregenerative refrigerators (AMRRs) for liquefying hydrogen process gas.In particular, disclosed herein are processes and systems that usehydrogen gas simultaneously for the process gas and the heat transferfluid. This decreases the capital cost of the liquefier by eliminatingan entire subsystem associated with separating a portion flow of a coldheat transfer fluid to form a bypass stream in counterflow with ahydrogen process fluid to be cooled and liquefied in process heatexchangers. The direct cooling of the hydrogen gas within highlyeffective active magnetic regenerators will increase the thermodynamicefficiency by use of much larger specific area (e.g., 4,000 to 10,000m²/m³) in magnetic regenerators to reduce temperature approach betweenmagnetic refrigerant and hydrogen heat transfer fluid and process fluid.In addition, for the high FOM hydrogen liquefier an ortho-para catalystcould be coated directly onto the magnetic material(s) in theregenerator in a core-shell configuration. For example, a dispersion offine catalyst particles can be directly adhered to magnetic refrigerantparticles via an adhesive. This will not substantially impact theactivity of the catalyst or the magnetocaloric effect of the refrigerantwhile allowing for faster, more efficient conversion of theortho-hydrogen to para-hydrogen during cooling which is required forefficient hydrogen liquefaction.

In certain embodiments, the process gas and the heat transfer gas bothcomprise hydrogen. In certain embodiments, the process gas and the heattransfer gas consist essentially of hydrogen (e.g., 98% or 99% hydrogen,with the remainder being non-condensing or freezing impurities such ashelium gas at LH₂ temperature).

At least three different classes of cryogenic AMRR designs have evolvedduring the past three decades that include: i) a reciprocating dualregenerator design with stationary superconducting magnet(s) or astationary dual regenerator design with reciprocating magnets in whichthe steps of the AMR cycle are executed sequentially in time; ii) astationary dual magnetic regenerator with a nested set of threeconcentric superconducting dipole magnets with the outer magnetencompassing both inner magnets which are connected in opposition via asuperconducting connection such that rotation of the outer dipolechanges the magnetic field in the inner two magnets alternatively fromvery low field to very high field in which the AMR cycle is alsoexecuted sequentially in time, and iii) a rotary wheel design withstationary superconducting magnets. The rotary wheel design with asection of the wheel in high magnetic field and an oppositely locatedidentical section in low magnetic field is inherently a dual regeneratordesign with continuous execution of all four steps of an AMR cycle atdifferent locations around the rim of the wheel. All of these designscan be used to implement the presently disclosed processes, but we havechosen the rotary wheel design to better illustrate the features of theprocesses and systems.

All configurations of the AMRR processes and systems have one or morelayers of magnetic refrigerants that execute active magneticregenerative cycles when coupled to a reversing flow of heat transfergas in the magnetized or demagnetized stages of the cycle. For optimalthermodynamic performance of cryogenic AMRRs with large temperaturespans for liquefaction, use of the difference in thermal mass (the massof refrigerant times it temperature and field heat capacity) offerromagnetic refrigerants below their respective Curie temperaturesenables use of bypass flow of cold heat transfer gas to continuouslycool a process stream and increase the FOM of the liquefier. Differentmass flow rates of heat transfer gas are required in magnetized vs.demagnetized steps of the AMR cycle due to the difference in total heatcapacity at the same temperature in high and low magnetic fields sectionof the wheel. For 20-40 K below the Curie temperature, the low-fieldheat capacity is as much as ˜10% larger than the high field heatcapacity to thermally balance (remove or add the same amount of energy)the dual regenerators during a complete steady-state AMR cycle so alarger flow of heat transfer fluid is required for the hot to cold flowthrough the demagnetized refrigerant step of the AMR cycle than in thecold to hot flow through the magnetized refrigerant step of the AMRcycle for the optimal performance of the AMRR cycle. The difference inflow rates can be accomplished by separating a portion of the cold heattransfer fluid after the hot-to-cold flow into a bypass stream that isseparately returned to the hot side of the AMRR, preferably through theprocess heat exchanger. In the presently disclosed processes where theheat transfer gas and the bypass gas are common, there is no need forexternal process heat exchangers because the bypass flow is coldhydrogen fluid (nominally at a pressure such 300 psia which is greaterthan hydrogen critical pressure of ˜180 psia) that is cooledsufficiently below hydrogen's critical temperature of ˜33 K by the AMRRso an expansion of the hydrogen fluid to lower pressure than hydrogen'scritical pressure results in conversion of the bypass portion of thecold hydrogen to 100 percent liquid hydrogen (no vapor return line isrequired).

In certain embodiments described herein, the systems and processes canprovide refrigeration between 280 K and 20 K with an apparatus utilizingrotary wheel regenerators comprised of layered ferromagnetic materialswith Curie temperatures between 293 K and 45 K or between 293 K and 53K.

To make a highly efficient liquefier for hydrogen, several featuresshould be used in its design. These features include:

-   -   Use an inherently efficient thermodynamic cycle;    -   Use an efficient work input device or mechanism;    -   Use an efficient work recovery device or mechanism;    -   Insure small temperature approaches for heat transfer between or        among streams or between solids and streams;    -   Use high specific area and highly-effective regenerative heat        exchangers;    -   Keep pressure drops for heat transfer gas flows and process gas        flow very low;    -   Invoke low longitudinal thermal conduction mechanisms via        material and geometry choices;    -   Minimize frictional and parasitic heat leak mechanisms; and    -   Continuous contact between hydrogen and active ortho to para        catalysts to ensure continuous conversion from ortho to para        during cooling of the fluid.

The processes and systems disclosed herein provide more efficienthydrogen liquefaction in an AMRL by combining hydrogen as the heattransfer fluid and the process gas in one or more AMRR stages (e.g.,1-3) compared to the same AMRR stages where the hydrogen process gas iscooled and liquefied separately from the magnetic regenerators in theAMRR stages. In the common hydrogen fluid design presently disclosedherein, the magnetic regenerators have 2-5 times more effective heattransfer than in counterflowing process heat exchangers although bothAMRL designs have continuous cooling of the hydrogen from ˜280 K to ˜23K for high FOM. The AMRL with common hydrogen streams as presentlydisclosed herein has fewer components as well because the separateprocess heat exchangers are eliminated. An AMRL disclosed hereinspanning from ˜280 K to ˜23 K can have designs such as 3 AMRR stageswith spans of ˜280 K to ˜120 K (8 refrigerants), ˜120 K to ˜50 K (3refrigerants), and ˜50 K to ˜23 K (2 refrigerants) or alternatively 1AMRR stage with 13 refrigerants). The ability to continuously cool asingle hydrogen process stream from near room temperature to itsliquefaction temperature uniquely creates a magnetocaloric hydrogenliquefier (MCHL) [also called an active magnetic regenerative liquefier(AMRL)] with much higher FOM than conventional gas-cycle liquefiers suchas a Claude cycle design that do not have the feature of bypass flow. Insuch AMRL designs rejection and absorption of heat are achieved by thetemperature increase or decrease of magnetic refrigerants inregenerators upon isentropic magnetization or demagnetization combinedwith reciprocating flow of heat transfer gas (hydrogen herein).

Certain embodiments of the novel processes and systems include the useof two dual rotary sets of identical layered active magneticregenerators with refrigerant masses sized to match to the coolingrequirements of GH₂ with hydrogen as a circulating common heat transfergas to continuously and simultaneously execute the 4-steps of the AMRcycles at different sections of rotating AMRR; i.e. as severalregenerator segments in the wheel rim are magnetized in the highmagnetic field region of a rotary AMRR, several others are demagnetizedin the low-magnetic field region. The regenerators of the rotaryconfiguration in the two magnetic field regions are oriented opposite toeach other such that circulating hydrogen heat transfer gas flows fromhot-to-cold temperatures in the demagnetized regenerator segments whilea different portion of hydrogen heat transfer fluid simultaneously flowsfrom cold-to-hot temperatures in the magnetized regenerator segments.For example, several segments of layered magnetic refrigerantsfabricated into tapered regenerators attached to a rotating wheel rimare magnetized simultaneously while another equal number of identicalregenerator segments are demagnetized. Similarly, the continuous flow ofhydrogen heat transfer gas establishes the hot-to-cold flow in severaldemagnetized regenerator segments at the same time as hydrogen heattransfer gas in an opposite part of the rotational path creates thecold-to-hot flow in several magnetized regenerator segments. The“several” regenerator segments executing the different steps of the AMRcycle comprise the “set”. Each regenerator in a given ‘set’ at anylocation around the wheel has an identical regenerator in another ‘set’executing the opposite step of the AMR cycle 180 degrees around therotating wheel. In this instance, there are several (e.g., 36) “dual”regenerators segments in the rotary wheel executing the opposite step inthe AMR cycle (see FIG. 1 ).

An AMR cycle has four steps that are being continuous executed at somelocation around the rim of the wheel: magnetization with no heattransfer fluid flow of one set of active magnetic regenerator segmentsas they move from low field to high field section of the wheel in FIG. 1; flow of heat transfer fluid (hydrogen in this case) from cold-to-hotthrough the magnetized regenerators; demagnetization of this same set ofsegments of regenerators while there is no flow of heat transfer fluid;and flow of heat transfer fluid from hot-to-cold through the segments ofdemagnetized regenerators. FIG. 1 illustrates that four sets of layeredregenerators in the rim of the wheel in the AMRR stage undergo the foursteps of the AMR cycle, but 180 degrees out of phase with a similar setof identical regenerator beds. After the AMRR in FIG. 1 has executedseveral hundred cycles at 1-2 Hz, i.e. in 10-15 minutes, the layeredactive magnetic regenerators in the rim of the wheel and throughout theother AMRR subsystems in FIG. 1 will have achieved constant steady-statetemperatures depending upon the location in the AMRR system. Eachregenerator segment will have dynamic temperatures as the segmentrotates through the different steps of the AMR cycle. There will be atemperature gradient across each layer of magnetic refrigerant that willspan from the average T_(HOT) to the average T_(COLD) along the flowaxis of the rotating active magnetic regenerators, i.e., in the radialor axial directions through the rim of the wheel in FIG. 1 .

The magnetic refrigerants in the AMR beds have a difference in thermalmass which is the product of heat capacity per unit mass times the massof magnetic refrigerant (or just heat capacity in this case because themass of magnetic material in a magnetic regenerator does not depend upontemperature or magnetic field). As illustrated in FIG. 2 the heatcapacity of a ferromagnetic material below its Curie temperature(ordering temperature) is smaller in higher magnetic fields than atlower or zero magnetic fields. However, as shown in FIG. 2 , thisdifference reverses in magnitude at the Curie temperature where the heatcapacity in higher magnetic fields becomes larger than the heat capacityin low or zero fields. This means the integral of the thermal mass foran AMR cycle spanning from 20-30 K below the Curie temperature to 20-30K above the Curie temperature increases up to the Curie temperature andthen it decreases with increasing temperature. The difference in thermalmass in ferromagnetic refrigerants below their respective Curietemperature creates the need for less hydrogen heat transfer fluid flowrate in the magnetized sector of the rotary AMRR than in thedemagnetized sector of a thermally balanced AMR cycle. Therefore, afterthe hot-to-cold flow of hydrogen through the demagnetized sector of thewheel, the hydrogen stream will be at an average temperature ofT_(COLD)−ΔT_(COLD)/2, and can preferably be split into the lesser amountof hydrogen heat transfer fluid required for cold-to-hot flow throughregenerator segments in the magnetized wheel sector and a smallerportion of cold hydrogen called bypass flow (so-called because thisportion bypasses the cold-to-hot flow through the set of magnetizedregenerators). In this embodiment, the cold bypass hydrogen portion at300 psia is expanded to create LH₂ at ˜35 psia that is collected in asmall storage vessel inside the evacuated cold box for transfer out ofthe AMRL into an external well-insulated storage tank (Dewar). In thedesign of the novel processes and systems disclosed the importance ofselecting and controlling the hot sink temperature and temperature spanfor each layer of magnetic refrigerants in the regenerators to maximizethe difference in thermal mass (and thereby the amounts of bypass flow)is recognized. First, the dynamic T_(HOT)±ΔT_(HOT) of the outer-mostrefrigerant in the layered rim of the wheel in FIG. 1 at its maximumduring the magnetization step of the AMR cycle is always less than its'Curie temperature. T_(HOT) is the environmental temperature where theheat is dumped. The dynamic T_(HOT) of the outer rim material rangesfrom T_(HOT)+ΔT_(HOTup) to T_(HOT)−ΔT_(HOTdown) during a complete AMRcycle where ΔT_(HOTup) is the increase in temperature caused byinserting the regenerator into the magnetic field and ΔT_(HOTdown) isthe decrease in temperature from T_(HOT) caused by removing aregenerator segment of the wheel from the magnetic field in the no-flowsector of the rotation. The maximum dynamic T_(HOT) depends on where itis in the cycle, but by design the maximum is T_(HOT)+ΔT_(HOT). This canbe done by setting a fixed heat sink temperature to anchor T_(HOT) whichin turn yields the largest difference in thermal mass between high andlow magnetic fields. The second aspect of the difference in thermal massin high and low magnetic fields is that it decreases steadily as thecold temperature in the particular layer of the regenerator decreasesbelow the Curie temperature (s) of the particular layer of magneticrefrigerant. Each independent magnetic refrigerant layer may be referredto herein as a “j^(th) layer”. Hence, the j^(th) layer of magneticmaterials in a regenerator in the subject AMRL for hydrogen, mustoperate in temperature spans when magnetized ofT_(HOTj)+ΔT_(HOTj)≤T_(Curiej) and T_(COLDj)+ΔT_(COLDj) equal to ˜20K<T_(Curiej) and when demagnetized, between T_(HOTj)−ΔT_(HOTj) andT_(COLDj)−ΔT_(COLDj) which are also ˜20 K apart. T_(COLDj) representsthe average cold temperature of the j^(th) slice of the magneticregenerators at any point in the AMRR as it executes the j^(th) smallmagnetic Brayton cycle. ΔT_(COLDj) represents the temperature dropcaused by the magnetocaloric effect when the regenerator is removed fromthe magnetic field. If larger temperature spans with optimum differencesin thermal mass are desired (as required for very high FOM), layers ofmagnetic materials with descending Curie temperatures must be used inthe AMRR regenerators.

For example, Gd metal is a ferromagnetic refrigerant that has a Curietemperature of about 293 K and is an option for the outer rim of thedisclosed embodiment. With ˜6.5 Tesla magnetic field changes [6.8 T to0.3 T], the adiabatic temperature change ΔT_(HOTup) (from low to highfield) starting from ˜280 K is about 12-13 K so T_(HOT)+ΔT_(HOT) duringits AMR cycle peaks at 280 K+12 K or 292 K. This temperature isindicated in FIG. 1 . The hot heat sink temperature in FIG. 1 enters theheat rejection counterflow heat exchanger at˜275 K and exits at˜279 K tocontinually cool the hydrogen heat transfer and process fluid to ˜280 Kbefore it enters the manifold for the hot-to-cold flow sector of thewheel. During a complete rotation of the wheel in FIG. 1 , Gd will bethe outermost layer of refrigerant in the rim-shaped layered regeneratorand its hot-side temperature will change from ˜292-293 K just as itenters the high field region to ˜280 K as it leaves the high fieldregion due to flow of hydrogen heat transfer fluid continuously enteringthe hot side of the wheel at ˜280 K. A similar shaped dynamictemperature cycle at the colder side of the Gd layer of the regeneratoris executed with an average T_(COLD) of ˜260 K. A similar dynamictemperature cycle is executed by different layers of magneticrefrigerants in the rim of the wheel to establish average temperaturesof 260 K to 240 K for the second layer (Gd_(0.83)Dy_(0.17) in thisembodiment) adjacent to the Gd layer, and 240 K to 220 K for the thirdlayer and so on through the multiple layers that comprise the rim of therotating wheel for a given stage of the AMRL. The temperature span ofeach layer is the difference between the average T_(HOT) and the averageT_(COLD) or about 20 K per layer. Because the thermal load of eachcolder layer of magnetic refrigerant is smaller than the next warmerlayer, the colder layer has less mass of refrigerant than the adjacentwarmer layer and therefore requires less hydrogen heat transfer fluidranging from the maximum in the outer-most layer to lesser flows in eachcolder layer. To adjust the proper flows per layer, a portion of theheat transfer fluid flowing from an outlet from the j^(th) layer in thehot-to-cold flow region is diverted via hermetic diversion flow channelsaround the circumference of the wheel into the cold-to-hot flow of heattransfer fluid inlet to the j^(th) layer in the cold-to-hot flow regionwith a controllable diversion valve to provide the optimum lesser flowfor the next colder demagnetized layer and simultaneously provideoptimum flow into the next hotter magnetized layer.

For example, in the rotary wheel embodiment, Gd is the outer layer andGd_(0.83)Dy_(0.17) is its adjacent colder layer. The mass of Gd isgreater than that of Gd_(0.83)Dy_(0.17) so after the optimum flow ofhydrogen heat transfer fluid passes through Gd in the hot-to-cold flow,a portion of it (˜10-˜50%) is separated as to provide optimum flow ofheat transfer and bypass fluid into the Gd_(0.83)Dy_(0.17) layer. Theportion of heat transfer fluid split off is the diversion flow from thethin interconnection gaps between the two layers mounted into the G-10regenerator housing. This flow is diverted circumferentially in ahermetic channel to the same gap between the Gd_(0.83)Dy_(0.17) layerand the Gd layer in the magnetized region of the wheel. The amount ofheat transfer fluid coming out of the Gd_(0.83)Dy_(0.17) layer is lessthan the optimum flow for the hotter Gd layer so the diversion flow isadded to it for optimum flow of heat transfer fluid into the Gd layerduring the cold-to-hot flow. The same thing happens between each pair ofadjacent successively colder layers until the inner-most layer.

For example, a first diversion flow comes from the small connection gapbetween a first layer and a second layer in the demagnetized region tobetween a second layer and a first layer in the magnetized region. Thebypass hydrogen fluid is a portion of the hydrogen fluid stream thatcontinues through all the layers to be separated into the expander afterthe hydrogen exits the coldest layer of magnetic refrigerant.

For each j^(th) layer of magnetic refrigerant, the value of T_(HOTj) isthe average of the dynamic temperature at the edge of the j^(th) layerin the rim of the wheel illustrated in FIG. 1 . The temperatures in/outof each layer through a multi-layer regenerator can be measured by tinytemperature sensors (such as a #36 gauge twisted-pair type Ethermocouples) inserted into the regenerator during fabrication of themonolithic regenerators. Measurements during rotation of the wheel willshow the small local magnetic Brayton cycles of each refrigerant thatare coupled into an AMR cycle by flow of the hydrogen heat transferfluid.

Illustrative embodiments of the AMRR stages are rotary designs with setsof dual regenerators that are simultaneously executing the four steps ofthe AMR cycle at all times during the rotational cycle of 1 to 2 Hertz.One embodiment shown, for example, in FIG. 1 , has four differentregions in the rotary design which allows continuous flow of thehydrogen heat transfer fluid through a set of demagnetized regenerators(a set of identical regenerators are in the low/zero field region) andsome magnetized regenerator beds (the identical regenerators in the highfield region) and no flow through the regions where the magnetic fieldis either increasing or decreasing. The LH₂ flow is continuouslyproduced from the expanded bypass portion of hydrogen heat transferfluid flow, and is compensated for by a make-up stream flowing from ahydrogen gas source at ˜280 K (which may be a pre-purified hydrogensource) into the hydrogen heat transfer fluid cycling flow just beforethe circulation pump for the combined hydrogen heat transfer and processfluid.

Certain embodiments of the novel processes and systems use layeredactive magnetic regenerators for enabling slightly larger differences as25-30 K between the average temperatures T_(HOT) and T_(COLD) necessaryto use fewer layers in hydrogen AMRL designs with higher or lowerpressure than the 300 psia illustrated in FIG. 1 . The magneticregenerators are fabricated with multiple longitudinally orradially-layered magnetic refrigerants located such that the Curietemperature of each refrigerant is above the average AMR-cycle hottemperature T_(HOT) by ΔT_(HOT) at that axial location in theregenerators in steady-state operation to maximize thermal massdifferences and thereby percentage of bypass flow. All the refrigerantsin the AMRR individually execute small magnetic Brayton cycles as theyare alternately magnetized and demagnetized by the magnetic field andconnected together from T_(HOT) to T_(COLD) by the flowing hydrogen heattransfer gas. This coupling allows the overall temperature span of anAMRR to be many times adiabatic temperature changes from themagnetocaloric effect of each magnetic refrigerant. The thermomagneticproperties of properly layered refrigerants must simultaneously haveentropy flows that satisfy the 2nd law of Thermodynamics (i.e.,ΔT_(COLDdown)=ΔT_(HOTup) (T_(COLD)/T_(HOT)) with allowance forgeneration of irreversible entropy and effects of parasitic heat leaksand bypass flow.

In certain embodiments, the active magnetic regenerative refrigeratorapparatus includes 1 to 16 layers, more particularly 1 to 13 layers, ofcompositionally distinct magnetic refrigerant materials in a singlestage that can, for example, span from ˜280 K to ˜20 K for production ofLH₂. In other embodiments, more than one AMRR stage can be connected inseries as shown, for example, in FIG. 3 . For instance, two AMRR stagesconnected in series can span from ˜280 K to ˜120 K with 8 layers ofdifferent refrigerants in the first stage and from ˜120 K to ˜20 K with5 layers of different refrigerants in the second stage with continuousflow of hydrogen heat transfer fluid for production of LH₂.

The active magnetic regenerative refrigerator apparatus comprises acomposition (e.g., a composite) that includes at least one magneticrefrigerant material and at least one ortho H₂ to para H₂ catalyst. Incertain embodiments, the composition comprises magnetic refrigerantmaterial particles (e.g., spheres) having a largest cross sectiondimension of up to 250 μm, and a binder interspersed with the particles,wherein the ortho H₂ to para H₂ catalyst is bonded to the particlesand/or the binder. In certain embodiments, the composition comprisesmagnetic refrigerant material particles having a largest cross sectiondimension of up to 250 μm, and ortho H₂ to para H₂ catalyst particleshaving a largest cross section dimension of less than 5 μm.

In certain embodiments, the magnetic refrigerant material is in the formof particles having a largest cross section dimension of up to 250 μm.For example, the magnetic refrigerant material particles may have adiameter of 150 to 250 μm. In certain embodiments, the magneticrefrigerant material particles may have a diameter of 100 to 250 μm. Theparticles may be spheres, non-spherical particles or powders, or rods.The particle may be of composite construction and is not necessarily apure substance. In certain embodiments, the magnetic refrigerantparticles may be spheres or similar high surface area per volumegeometries (e.g., specific areas of 5,000 to 10,000 m² per m³) such asparallel sheets or perforated plates.

The binder is a material that is disposed among and/or between themagnetic refrigerant particles without reducing the heat transfer rateto the particle while only slightly increasing the pressure drop, i.e.,1%, for low pressure-drop hydrogen flow through the regenerator layers.The binder provides structural integrity to each magnetic refrigerantcomposition so that it can be formed into layers. The binder may includeat least one polymer selected from polyethylene, ethylene vinyl acetate,polypropylene, polystyrene, polycarbonate, an epoxy resin, or apolyurethane resin. The binder may be an adhesive agent. In certainembodiments, the binder comprises at least one curable epoxy resin thatis applied in dilute solutions of the epoxy-hardener mixture diluted by10:1 times by volume with a volatile solvent such as acetone. In certainembodiments, the magnetic refrigerant particles may be dispersed in amatrix of the binder.

An optimal loading of a highly active catalyst with the magneticrefrigerants enables continual ortho-to-para conversion of hydrogen atthe highest possible temperature which enhances liquefier efficiency. Incertain embodiments, the catalyst loading is 0.1-10 wt %, moreparticularly 0.1-5 wt %, based on the weight of the refrigerant perlayer. One of the significant benefits of the systems and processesdisclosed herein is elimination of the external counterflow process gasheat exchangers by excellent internal heat exchange inherent in veryhigh surface area magnetic regenerators.

In certain embodiments, the magnetic refrigerant material particles arecontacted with the binder. The binder is then cured. The cured binderconnects the particles at their contact points but retains porosity forflow of hydrogen heat transfer fluid through the bed with low pressuredrop. To avoid reducing thermal conductance between the hydrogen heattransfer fluid and the particles, any coating on the particles should bethin (e.g., less than 5 microns thick, more particularly less than 2microns thick). After the binder is cured, the ortho-to-para catalystsare separately bonded to the high surface area of the resulting bondedmagnetic refrigerant material particles to avoid reducing the activityof the catalysts. The catalyst may be bonded to the particles viaphysical adsorption means that does not reduce the catalyst activity;e.g., this could be by a short thermal diffusion process after theinitial binder is added and cured on the magnetic particles. It can alsobe done by washcoating as described below.

Illustrative ortho H₂ to para H₂ catalysts for use in the bypass flowprocess heat exchangers include, but are not limited to, activatedcarbon; ferric oxide (Fe₂O₃); chromic oxides (Cr₂O₃ or CrO₃); Ni metaland Ni compounds (Ni²⁺); rare earth metals and oxides such as Gd₂O₃,Nd₂O₃, and Ce₂O₃; Pt; and Ru. Activated carbon and ferric oxide areparticularly preferred. The catalysts can be directly coated onto themagnetic refrigerant materials in the regenerators. If themagnetocaloric materials in the regenerators are not secured in place byan epoxy or other coating method, a small amount of catalysts can bemixed in with the active magnetocaloric materials. The catalystparticles may be the same or similar in size and shape as themagnetocaloric materials, or the catalyst particles may be smaller insize but similar in shape as the magnetocaloric materials. If themagnetocaloric materials are secured in place for example by an epoxyprocess then the catalyst will need to be coated onto the surface of thematerials after the diluted epoxy is applied and cured. This can be doneusing standard washcoating process followed by a reduction and mildoxidation as needed. For example, a solution of iron hydroxide [Fe(OH)₃]in a basic solution (pH 8-10), at the desired weight percent, can bewash coated onto the magnetic refrigerant material. The coated materialscan be dried at mild temperatures (<100-120° C.) which dehydrates toFe₂O₃. For this catalyst no reduction is needed. Similar washcoatingtechniques can be used for the other catalysts. However, the solvents,drying temperatures, and, if required, reduction procedures must becompatible with the binder and the magnetocaloric materials. Forexample, washing with NH₃BH₃, NaBH₄ or N₂H₂ can be used to reduce themetal oxides to metals.

Gadolinium is an excellent magnetic refrigerant and has been generallyaccepted as the reference material against which other refrigerants arecompared. It has a simple ferromagnetic ordering temperature of ˜293 Kand exhibits an adiabatic temperature change of ˜2 K/Tesla overpractical magnetic field strengths (up to ˜8 T). It also has a largedifference in field-dependent thermal mass just below its Curietemperature as shown in FIG. 2 . Introduction of alloying additions ofanother lanthanide metal reduces the magnetic-ordering temperature of Gdwithout much effect on the total magnetic moment per unit volume and thechange in magnetization with temperature near a sharp orderingtemperature.

For example, homogeneous alloys of Gd with other rare earth metals (Tb,Er, Dy, Ho) or Y make superior magnetic refrigerants. Some elementalrare earth materials such as Ho and Er have more complex magneticordering phenomenon but when alloyed with Gd these effects tend to bereduced at high magnetic fields to provide acceptable ferromagnetic orheliomagnetic refrigerants. The addition of non-magnetic Y to Gd forms ahomogeneous alloy with a reduction in adiabatic temperature changecompared to Gd but simultaneously decreases the magnetic orderingtemperature and exhibits simple ferromagnetism down to about 230 K.

Key features or suitable refrigerant materials include:

-   -   Use ferromagnetic materials that operate below their Curie        temperature throughout their entire AMR cycle;    -   Maintain average T_(HOT) at least ΔT_(HOT) below the Curie        temperature of the j^(th) layer of magnetic material in a        regenerator; this applies to each layer of magnetic material in        the regenerator with correspondingly lower cycle temperatures;    -   Average temperature difference between T_(HOT) and T_(COLD) of        each layer of magnetic refrigerant should be ˜20 K per layer;    -   Spanning from 280 K to 120 K in one AMRR stage requires 8        refrigerants to be combined into optimally layered regenerators.    -   Layering must have smooth flows of energy and entropy at        transitions between layered refrigerants along the longitudinal        axis of the regenerator.

Illustrative magnetic refrigerants include those shown below in Table 1.

Operating Ordering Temperature Span Temperature Material K K Gd 280-260293 Gd_(0.90)Y_(0.10) 260-240 274 Gd_(0.30)Tb_(0.70) 240-220 253Gd_(0.69)Er_(0.31) 220-200 232 Gd_(0.02)Tb_(0.98) 220-200 233Gd_(0.32)Dy_(0.68) 200-180 213 Gd_(0.66)Y_(0.34) 200-180 213Gd_(0.39)Ho_(0.61) 180-160 193 Gd_(0.59)Y_(0.41) 180-160 193Gd_(0.15)Dy_(0.85) 180-160 193 Gd_(0.42)Er_(0.58) 160-140 173Gd_(0.27)Ho_(0.73) 160-140 173 Gd_(0.16)Ho_(0.84) 140-120 153Gd_(0.34)Er_(0.66) 140-120 152 Gd_(0.23)Er_(0.77) 120-100 132(Ho₀.₈₀Gd_(0.20))Co₂ 120-100 130 Ho_(0.90)Gd_(0.10)Co₂ 100-80  110Ho_(0.95)Gd_(0.05)Co₂ 80-60 90 Gd_(0.5)Dy_(0.5)Ni₂ 60-40 70Dy_(0.75)Er_(0.25)Al₂ 40-20 50 (Gd_(x)Er_(1−x))Al₂ 150-10  168 (x = 1)tp 15 (x = 0)

As shown in FIG. 1 , the rotary AMRR apparatus includes an annular bed 1of at least several porous magnetic refrigerant material-containingcompositions. The bed 1 may include a plurality of layers (e.g., 13, or14, or at least 5, or at least 8) wherein each layer is compositionallydistinct from each other layer. The magnetic refrigerantmaterial-containing composition may include ortho-to-para catalystsbonded to magnetic refrigerant material surfaces for maximum catalyticactivity at the highest possible temperatures in the regenerators. It isthe continuous range of temperatures from 280 K to 20 K in the layeredregenerator where any cooled ortho hydrogen is converted to parahydrogen to maintain the equilibrium ratio of ortho to para hydrogen(75:25 ortho:para at 280 K to 0.2:99.8 at 20 K).

The rotary AMRR apparatus that is physically the same everywhere in thewheel is operationally divided into four sections during execution ofthe AMR cycle (listed in order of wheel rotation): (i) a high magneticfield section in which the hydrogen heat transfer fluid (e.g., heattransfer gas) flows from a cold side to a hot side through themagnetized regenerators(s) bed(s), (ii) a first no heat transfer fluid(e.g., gas) flow section in which the regenerator(s) bed(s) aredemagnetized, (iii) a low magnetic or demagnetized field section inwhich the hydrogen heat transfer fluid (e.g., gas) flows from a hot sideto a cold side through the demagnetized regenerator(s) bed(s), and (iv)a second no hydrogen heat transfer fluid (e.g., gas) flow section inwhich the regenerator(s) bed(s) are magnetized. Transverse andcircumferential seals are provided in the no heat transfer fluid flowsections to prevent undesirable hydrogen heat transfer fluid flow. Themagnetic refrigerant beds or segments may be divided into compartmentswherein the compartments contain layered magnetic refrigerants (e.g., 13or 14 layers) identical to those in other segments around the wheel rim.

The rotary AMRR apparatus includes a rotating wheel that includes aninside hollow annular rim 2 (inner housing and flow duct wall) and anoutside hollow annular rim 3 (outer housing and flow duct wall). A hothydrogen heat transfer fluid (HTF) is introduced into the outside rim 3of the rotary AMRR apparatus via an HTF inlet duct provided in the lowmagnetic or demagnetized field section (iii). The hot HTF in the outsiderim 3 has a steady-state circumferentially average temperature that, forexample, may be 280 K. This is a fixed temperature that is controlled byadjusting a chiller temperature feeding the heat rejection heatexchanger that insures hydrogen fluid enters the outer rim of thedemagnetized regenerator compartments in the hot to cold flow region ofthe wheel at a temperature ˜12-13 K below the Curie temperature of Gd,the outer most layer in the wheel rim. However, the local temperature ata given time and location in the AMR cycle may differ from thesteady-state circumferentially average temperature. The hot HTF flows ina radial direction through the demagnetized bed in the low magneticfield region, cooling the hydrogen combined process and heat transferfluid. Optimum flow of cooled heat transfer fluid exits the low magneticor demagnetized field section (iii) via an HTF outlet duct and into theinside rim 2. The HTF radial flow is shown by the arrows 4 in the lowmagnetic or demagnetized field section (iii). The cold HTF in the insiderim 2 has a steady-state circumferentially average temperature that, forexample, may be ˜20-23 K. However, the local temperature at a given timeand location in the AMR cycle may differ from the steady-statecircumferentially average temperature.

The inside rim 2 is fluidly coupled via an HTF outlet duct and a conduit7 to a conduit junction 9. The hydrogen heat transfer fluid exiting thecold side of the low magnetic or demagnetized field section is at atemperature of 20 to 23 K and a pressure of about 300 psia. The conduitjunction is fluidly coupled via a conduit 10 to an expander 8. A bypassportion of the hydrogen heat transfer fluid is separated into theexpander 8 via conduit 10. The expander 8 isenthalpically expands thebypass portion of the hydrogen heat transfer fluid from ˜300 psia to˜15-35 psia, more particularly 35 psia, to produce liquefied hydrogen(LH₂). The liquefied hydrogen exiting the expander is at a temperatureof 20 to 25 K, more particularly 23 K, and a pressure of 15 to 35 psia,more particularly 35 psia. In certain embodiments, the LH₂ can bedelivered directly to a storage tank, for example, a storage tankexternal of the system. The flow at the junction 9 may be controlled aLH₂ flow control valve (not shown). In certain embodiments, 3-12%,particularly less than 12%, more particularly less than 8%, and mostparticularly 6%, of the hydrogen heat transfer fluid is separated asbypass flow to the expander 8. The remaining hydrogen heat transferfluid is introduced as the cold flow into the inside rim 2 at the highmagnetic field section (i) via conduit 11 and an HTF inlet duct.

The cold HTF flows in a radial direction through the high magnetizedbed, heating the HTF. The hot HTF exits the high magnetic field section(i) via an HTF outlet duct and into the outside rim 3. The HTF radialflow is shown by the arrows 5 in the high magnetic field section (i).The hot HTF exits the high magnetic field section (i) and is introducedvia a conduit 12 into a circulation pump and then into a hot heatexchanger (HHEX). The HHEX cools the heat transfer fluid down to asuitable temperature close to 280 K for introduction as the hot flowinto the low magnetic or demagnetized field section (iii). The coolingheat transfer fluid in the HHEX typically is a water/glycol mixturechilled to ˜278 or 275 K to ensure the hydrogen HTF is at 280 K beforeit flows in the hot to cold flow section of the wheel. In certainembodiments, the hydrogen heat transfer fluid enters to HHEX at 292 to286 K and 290 to 300 psia. In certain embodiments, the hydrogen heattransfer fluid entering as the hot flow into the low magnetic ordemagnetized field section (iii) is at 280 K and 300 psia.

A hydrogen gas source is also provided. In the embodiment shown in FIG.1 , hydrogen gas (GH₂) from a hydrogen gas source is introduced viaconduit 13 into the hydrogen heat transfer fluid flowing from the hotside of the high magnetic field section into the hot side of the lowmagnetic or demagnetized field section. In certain embodiments, theamount of GH₂ introduced via conduit 13 compensates the mass flow in thehydrogen heat transfer fluid cycle for the amount diverted for LH₂production. In certain embodiments, the GH₂ is at 280 K and 300 psia.

In certain embodiments, the rim of the wheel has multiple layers (e.g.,16, more particularly 14 or 13) of different ferromagnetic refrigerantswith Curie temperatures about 20 K apart between successively adjacentlayers in the direction from inner layer to the outer layer with thecoldest layer in the inside most layer on the rim of the wheel to haveCurie temperatures from ˜293 K to ˜33 K. In certain embodiments, themagnetic refrigerant materials are arranged in descending order from theouter layer to the inner layer according to Curie temperatures. Theouter layer is near room temperature and rejects heats into a thermalsink and the inner-most layer cools the hydrogen fluid to slightly below˜23 K such that 6-10% bypass portion, for example ˜10%, bypass portionof hydrogen can be isenthalpically expanded to produce LH₂ at ˜35 psiaand also absorb the small intrinsic parasitic heat leaks into the AMRR.

The hydrogen gas source may be, for example, an electrolyzer, a steammethane reformer, or a methane autoreformer. Typical hydrogen feedstockpressure from an electrolyzer, a steam methane reformer, or a methaneautoreformer is about 300 psia. The critical pressure of hydrogen is12.2 atm or 197.2 psia so the hydrogen is a single-phase fluid at 300psia. This pressure is an excellent heat transfer fluid pressure for anAMRR from a mechanical design perspective. In the AMRR of the presentlydisclosed process the hydrogen ‘fluid’ is both the heat transfer mediumand the process medium. The hydrogen is efficiently cooled in the highlyeffective dual magnetic regenerators of the AMRR to a cold temperatureof ˜23 K selected such that expansion of the bypass portion of the heattransfer stream from ˜300 psia to ˜35 psia will produce 100% LH₂ that istransferred to a LH₂ storage tank at a plant. Hydrogen as a heattransfer gas is superior to helium gas so the AMRR thermodynamicperformance (FOM) in the presently disclosed processes should also beimproved. The mass flow rate of the LH₂ leaving the AMRR is continuouslymade up from the external hydrogen source at the inlet to the heattransfer gas pump at room temperature.

Certain embodiments of the processes and system disclosed hereindescribed in the following numbered clauses:

1. A process for liquefying a hydrogen process gas comprising:

introducing a hydrogen heat transfer fluid into an active magneticregenerative refrigerator apparatus that comprises (i) a high magneticfield section in which the hydrogen heat transfer fluid flows from acold side to a hot side through at least one magnetized bed of at leastone magnetic refrigerant, (ii) a first no heat transfer fluid flowsection in which the bed is demagnetized, (iii) a low magnetic ordemagnetized field section in which the hydrogen heat transfer fluidflows from a hot side to a cold side through the demagnetized bed, and(iv) a second no heat transfer fluid flow section in which the bed ismagnetized;

continuously introducing the hydrogen heat transfer fluid from the coldside of the low magnetic or demagnetized field section into the coldside of the high magnetic field section;

continuously diverting a portion of the hydrogen heat transfer fluidflowing from the cold side of the low magnetic or demagnetized fieldsection into an expander; and

isenthalpically expanding the diverted portion of the hydrogen heattransfer fluid to produce liquefied hydrogen.

2. The process of clause 1, wherein the diverted portion constitutes 3to 12% of the total hydrogen heat transfer fluid exiting the cold sideof the low magnetic or demagnetized field section.

3. The process of clause 1 or 2, wherein the magnetic refrigerantoperates at or below its Curie temperature throughout an entire activemagnetic regeneration cycle.

4. The process of any one of clauses 1 to 3, wherein the processprovides a figure of merit (FOM) of at least 0.5.

5. The process of any one of clauses 1 to 4, wherein the active magneticregenerative refrigerator apparatus includes a plurality of magneticrefrigerant layers.

6. The process of clause 5, wherein the active magnetic regenerativerefrigerator apparatus includes 1 to 16 layers of compositionallydistinct magnetic refrigerant materials.

7. The process of any one of clauses 1 to 6, wherein the active magneticregenerative refrigerator apparatus comprises a composition thatincludes at least one magnetic refrigerant material and at least oneortho H₂ to para H₂ catalyst.

8. The process of clause 7, wherein the magnetic refrigerant material isin the form of particles having a largest cross section dimension of upto 250 μm.

9. The process of clause 7, wherein the composition comprises magneticrefrigerant material particles having a largest cross section dimensionof up to 250 μm, and a binder interspersed with the particles, whereinthe ortho H₂ to para H₂ catalyst is bonded to the particles and/or thebinder.

10. The process of clause 7, wherein the composition comprises magneticrefrigerant material particles having a largest cross section dimensionof up to 250 μm, and ortho H₂ to para H₂ catalyst particles having alargest cross section dimension of less than 5 μm.

11. The process of any one of clauses 8 to 10, wherein the magneticrefrigerant material particles have a diameter of 150 to 250 μm.

12. The process of clause 9, wherein the binder comprises at least oneepoxy material.

13. The process of any one of clauses 1 to 12, wherein the magneticrefrigerant material is selected from Gd, Gd_(0.90)Y_(0.10),Gd_(0.30)Tb_(0.70), Gd_(0.69)Er_(0.31), Gd_(0.02)Tb_(0.98),Gd_(0.32)Dy_(0.68), Gd_(0.66)Y_(0.34), Gd_(0.39)Ho_(0.61),Gd_(0.59)Y_(0.41), Gd_(0.15)Dy_(0.85), Gd_(0.42)Er_(0.58),Gd_(0.27)Ho_(0.73), Gd_(0.16)Ho_(0.84), Gd_(0.34)Er_(0.66),Gd_(0.23)Er_(0.77), (Ho_(0.80)Gd_(0.20))Co₂, Ho_(0.90)Gd_(0.10)Co₂,Ho_(0.95)Gd_(0.05)Co₂, Gd_(0.5)Dy_(0.5)Ni₂, or Dy_(0.75)Er_(0.25)Al₂.

14. The process of any one of clauses 1 to 12, wherein the magneticrefrigerant material is a material with a second order phase transition.

15. The process of any one of clauses 1 to 14, wherein the liquefiedhydrogen exiting the expander is at a temperature of 20 to 23 K and apressure of 15 to 35 psia.

16. The process of any one of clauses 1 to 15, wherein the hydrogen heattransfer fluid exiting the cold side of the low magnetic or demagnetizedfield section is at a temperature of 20 to 23 K and a pressure of 300psia.

17. The process of any one of clauses 1 to 16, further comprisingcontinuously introducing the hydrogen heat transfer fluid from the hotside of the high magnetic field section into the hot side of the lowmagnetic or demagnetized field section.

18. The process of clause 17, further comprising introducing hydrogengas from a hydrogen gas source into the hydrogen heat transfer fluidflowing from the hot side of the high magnetic field section into thehot side of the low magnetic or demagnetized field section.

19. The process of any one of clauses 1 to 18, wherein the hydrogen heattransfer fluid consists essentially of hydrogen.

20. A system comprising:

an active magnetic regenerative refrigerator apparatus that comprises(i) a high magnetic field section in which a hydrogen heat transferfluid can flow from a cold side to a hot side through at least onemagnetized bed of at least one magnetic refrigerant, (ii) a first noheat transfer fluid flow section in which the bed can be demagnetized,(iii) a low magnetic or demagnetized field section in which the hydrogenheat transfer fluid can flow from a hot side to a cold side through thedemagnetized bed, and (iv) a second no heat transfer fluid flow sectionin which the bed can be magnetized;

a first conduit fluidly coupled between the cold side of the lowmagnetic or demagnetized field section and the cold side of the highmagnetic field section; and

a second conduit fluid coupled to the first conduit, an expander and atleast one liquefied hydrogen storage module.

21. The system of clause 20, wherein the active magnetic regenerativerefrigerator apparatus comprises a composition that includes at leastone magnetic refrigerant material and at least one ortho H₂ to para H₂catalyst.

22. The system of clause 21, wherein the magnetic refrigerant materialis in the form of particles having a largest cross section dimension ofup to 250 μm.

23. The system of clause 21, wherein the composition comprises magneticrefrigerant material particles having a largest cross section dimensionof up to 250 μm, and a binder interspersed with the particles, whereinthe ortho H₂ to para H₂ catalyst is bonded to the particles and/or thebinder.

24. The system of clause 21, wherein the composition comprises magneticrefrigerant material particles having a largest cross section dimensionof up to 250 μm, and ortho H₂ to para H₂ catalyst particles having alargest cross section dimension of less than 5 μm.

25. The system of any one of clauses 22 to 24, wherein the magneticrefrigerant material particles have a diameter of 150 to 250 μm.

26. The system of clause 23, wherein the binder comprises at least oneepoxy material.

27. The system of any one of clauses 20 to 26, wherein the magneticrefrigerant material is selected from Gd, Gd_(0.90)Y_(0.10),Gd_(0.30)Tb_(0.70), Gd_(0.69)Er_(0.31), Gd_(0.02)Tb_(0.98),Gd_(0.32)Dy_(0.68), Gd_(0.66)Y_(0.34), Gd_(0.39)Ho_(0.61),Gd_(0.59)Y_(0.41), Gd_(0.15)Dy_(0.85), Gd_(0.42)Er_(0.58),Gd_(0.27)Ho_(0.73), Gd_(0.16)Ho_(0.84), Gd_(0.34)Er_(0.66),Gd_(0.23)Er_(0.77), (Ho_(0.80)Gd_(0.20))Co₂, Ho_(0.90)Gd_(0.10)Co₂,Ho_(0.95)Gd_(0.05)Co₂, Gd_(0.5)Dy_(0.5)Ni₂, or Dy_(0.75)Er_(0.25)Al₂.

28. The system of any one of clauses 20 to 26, wherein the magneticrefrigerant material is a material with a second order phase transition.

29. A method for making a magnetic refrigerant composition, comprising:

contacting magnetic refrigerant material particles having a largestcross section dimension of up to 250 μm with a binder;

curing the binder; and

bonding an ortho H₂ to para H₂ catalyst to the bonded magneticrefrigerant material particles.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention.

What is claimed is:
 1. A process for liquefying a hydrogen gascomprising: introducing a hydrogen heat transfer fluid into an activemagnetic regenerative refrigerator apparatus that comprises (i) a highmagnetic field section in which the hydrogen heat transfer fluid flowsfrom a cold side to a hot side through at least one magnetized bed of atleast one magnetic refrigerant, (ii) a first no heat transfer fluid flowsection in which the bed is demagnetized, (iii) a low magnetic ordemagnetized field section in which the hydrogen heat transfer fluidflows from a hot side to a cold side through the demagnetized bed, and(iv) a second no heat transfer fluid flow section in which the bed ismagnetized; continuously introducing the hydrogen heat transfer fluidfrom the cold side of the low magnetic or demagnetized field sectioninto the cold side of the high magnetic field section; continuouslyseparating a bypass portion of the cold hydrogen heat transfer fluidflowing from the cold side of the low magnetic field or demagnetizedsection into an expander; and isenthalpically expanding the separatedportion of the hydrogen heat transfer fluid to produce liquefiedhydrogen.
 2. The process of claim 1, wherein the bypass portionconstitutes 3 to 12% of the total hydrogen heat transfer fluid exitingthe cold side of the low magnetic or demagnetized field section.
 3. Theprocess of claim 1, wherein the magnetic refrigerant operates at orbelow its Curie temperature throughout an entire active magneticregeneration cycle.
 4. The process of claim 1, wherein the processprovides a figure of merit (FOM) of at least 0.5.
 5. The process ofclaim 1, wherein the active magnetic regenerative refrigerator apparatusincludes a plurality of magnetic refrigerant layers.
 6. The process ofclaim 1, wherein the active magnetic regenerative refrigerator apparatusincludes 1 to 16 layers of compositionally distinct magnetic refrigerantmaterials.
 7. The process of claim 5, wherein the active magneticregenerative refrigerator apparatus includes up to 13 layers ofcompositionally distinct magnetic refrigerant materials.
 8. The processof claim 1, wherein the active magnetic regenerative refrigeratorapparatus comprises a composition that includes at least one magneticrefrigerant material and at least one ortho H₂ to para H₂ catalyst. 9.The process of claim 8, wherein the magnetic refrigerant material is inthe form of particles having a largest cross section dimension of up to250 μm.
 10. The process of claim 8, wherein the composition comprisesmagnetic refrigerant material particles having a largest cross sectiondimension of up to 250 μm, and a binder interspersed with the particles,wherein the ortho H₂ to para H₂ catalyst is bonded to the particlesand/or the binder.
 11. The process of claim 8, wherein the compositioncomprises magnetic refrigerant material particles having a largest crosssection dimension of up to 250 μm, and ortho H₂ to para H₂ catalystparticles having a largest cross section dimension of less than 5 μm.12. The process of claim 9, wherein the magnetic refrigerant materialparticles have a diameter of 150 to 250 μm.
 13. The process of claim 9,wherein the magnetic refrigerant material particles have a diameter of100 to 250 μm.
 14. The process of claim 10, wherein the binder comprisesat least one epoxy material.
 15. The process of claim 1, wherein themagnetic refrigerant material is selected from Gd, Gd_(0.90)Y_(0.10),Gd_(0.30)Tb_(0.70), Gd_(0.69)Er_(0.31), Gd_(0.02)Tb_(0.98),Gd_(0.32)Dy_(0.68), Gd_(0.66)Y_(0.34), Gd_(0.39)Ho_(0.61),Gd_(0.59)Y_(0.41), Gd_(0.15)Dy_(0.85), Gd_(0.42)Er_(0.58),Gd_(0.27)Ho_(0.73), Gd_(0.16)Ho_(0.84), Gd_(0.34)Er_(0.66),Gd_(0.23)Er_(0.77), (Ho_(0.80)Gd_(0.20))Co₂, Ho_(0.90)Gd_(0.10)Co₂,Ho_(0.95)Gd_(0.05)Co₂, Gd_(0.5)Dy_(0.5)Ni₂, or Dy_(0.75)Er_(0.25)Al₂.16. The process of claim 1, wherein the magnetic refrigerant material isGd_(0.83)Dy_(0.17), or (Gd_(x)Er_(1-x))Al₂, wherein x is 0 or
 1. 17. Theprocess of claim 1, wherein the magnetic refrigerant material is amaterial with a second order phase transition.
 18. The process of claim1, wherein the liquefied hydrogen exiting the expander is at atemperature of 20 to 23 K and a pressure of 15 to 35 psia.
 19. Theprocess of claim 1, wherein the hydrogen heat transfer fluid exiting thecold side of the low magnetic or demagnetized field section is at atemperature of 20 to 23 K and a pressure of 300 psia.
 20. The process ofclaim 1, further comprising continuously introducing the hydrogen heattransfer fluid from the hot side of the high magnetic field section intoa heat exchanger and then into the hot side of the low magnetic ordemagnetized field section.
 21. The process of claim 20, furthercomprising introducing hydrogen gas from a hydrogen gas source into thehydrogen heat transfer fluid flowing from the hot side of the highmagnetic field section into the hot side of the low magnetic ordemagnetized field section.
 22. The process of claim 1, wherein thehydrogen heat transfer fluid consists essentially of hydrogen.
 23. Theprocess of claim 5, wherein each j^(th) magnetic refrigerant layerincludes a heat transfer fluid outlet and a heat transfer fluid inletand the active magnetic regenerative refrigerator apparatus is in theshape of a circular wheel, the process further comprising diverting aportion of the heat transfer fluid flowing from the heat transfer fluidoutlet from the j^(th) heat transfer fluid layer in the hot-to-cold flowregion via hermetic diversion flow channels around the circumference ofthe wheel into the cold-to-hot flow of the heat transfer fluid inlet tothe j^(th) layer in the cold-to-hot flow region with a controllablediversion valve to provide lesser flow for the next colder demagnetizedlayer and simultaneously provide flow into the next hotter magnetizedlayer.